1. Field of the Invention
The present invention relates to an assembly for initiating explosives comprising a percussion-actuated detonator and a low-energy detonating cord (LEDC) adjacent the detonator's percussion-responsive end for the actuation thereof. The invention also relates to a percussion-actuated detonator provided with means for holding LEDC adjacent its percussion-responsive end.
2. Description of the Prior Art
Detonating cords are used in non-electric blasting systems to convey or conduct a detonation wave to an explosive charge in a borehole from a remote area. One type of detonating cord, known as low-energy detonating cord (LEDC), has an explosive core loading of only about 0.1 to 2 grams per meter of cord length. Such a cord is characterized by low brisance and the production of little noise, and therefore is particularly suited for use as a trunkline in cases where noise has to be kept to a minimum, and as a downline for the bottom-hole priming of an explosive charge.
In blasting practice, an LEDC downline may be joined to an instantaneous or delay detonator attached to the blasting explosive charge, or to an explosive primer in said charge, in a borehole. Detonation of the LEDC actuates the detonator, which in turn initiates the blasting explosive charge or primer. The more sensitive the blasting explosive charge, the lower the explosive loading of the LEDC has to be to avoid detonation of the blasting charge before actuation of the detonator. With some blasting explosives, a cord loading as low as about 0.5 g/m or less may be desired.
At the surface, a delay detonator may be interposed between two lengths of LEDC trunkline to provide a surface delay. Also, if the LEDC is of a type which is incapable of "picking up", i.e., detonating, from the detonation of a donor cord with which it is spliced or knotted, e.g., to connect downlines to a trunkline, an instantaneous or delay detonator may be interposed between the trunkline and downline to act as a "starter" for the downline.
The most desirable cord-initiated detonators are those which do not require connection to the cord at the place of manufacture. A field-assembled detonator/cord system offers such advantages as safety and convenience during handling and storage, possible separate classification of the components for transportation, etc.
U.S. Pat No. 4,335,652, issued June 22, 1982, describes a delay detonator adapted to be assembled in the field with a length of LEDC which is placed in coaxial position in an open cavity in the detonator, thereby making the detonator particularly useful as an in-hole delay initiator when connected to an LEDC downline. In this assembly the detonator is initiated by the exposed end of the cord.
U.S. Pat. No. 4,299,167, issued Nov. 10, 1981, describes an initiator for introducing a delay between two lengths of LEDC trunkline or an LEDC trunkline and LEDC downline. This surface delay initiator is actuated from the side output of a donor cord, and end-initiates a receiver cord. The donor cord is engaged in a transverse slot in a tubular connector having a bore for receiving the initiator.
U.S. Pat. No. 3,709,149 also describes a delay detonator adapted to be assembled in the field with a length of LEDC, the cord in this case being disposed outside a closed shell that contains an impact-sensitive ignition composition held, for example, in an empty primed rim-fired or center-fired rifle cartridge casing used as an end closure for the detonator. The end or side of the cord is in direct and abutting contact with the exterior surface of the primer end, thereby permitting utilization of either the side or end output of the cord for ignition. This detonator generally is positioned in a booster unit embedded in an explosive charge in a borehole.
Among percussion-actuated detonators, those having a partially empty, tubular metal primer shell, e.g., a primed rifle cartridge casing, as the percussion-responsive element are preferred on the basis of convenience of manufacture, accessibility of components, etc. With respect to cord orientation in LEDC/detonator assemblies, placement of the cord transverse to the axis of the detonator shell is preferred over a coaxial orientation, which requires that the cord be cut to provide an abutting end surface. However, regardless of whether or not the primer charge in the primer shell is at the center or along the rim of the end of the shell, the transversely oriented LEDC must be carefully placed and maintained against the end of the primer shell if the primer charge is to be ignited reliably by the cord's detonation. Especially with cords having explosive loadings below about 1.0 g/m, the proper relationship between the cord and the outside surface of the primer shell at the time of cord detonation is critical in view of the fact that the cord's initiation impulse must be transmitted through the side wall of the cord (e.g., a protective covering of plastic, woven textiles, etc.) and the end of the primer shell. During field assembly it is possible that the side of the cord may not properly abut the primer end surface, or that a foreign substance may become lodged between them. Also, the orientation of cord and primer surface may be disturbed during ensuing operations to prepare for blasting.
Therefore, the art has been in need of a means of achieving reliable actuation under field assembly conditions of detonators in which a primer charge in a partially empty, tubular metal primer shell is to be initiated by the side-output of a low-energy detonating cord.